This invention relates to an improved process for molding articles. More specifically, this invention relates to a molding process comprising an improved mold-coating step that allows the preparation of several molded articles after a single application of a curable silicone release composition to the mold.
The molding of solid articles is a highly successful commercial activity; however, it is an activity with some technical problems.
A prominent technical problem is the tendency of the molded article to stick to the shape-determining surfaces of the mold, thereby interfering with its separation from the mold. This sticking problem is particularly prevalent in molding processes which use liquid, organic molding compositions which are curable to the solid state, such as polyurethanes, unsaturated polyesters and polyepoxides. The liquid state of the molding composition allows it to achieve intimate contact with, and even wet, the shape-determining surfaces of the mold, thereby favoring adhesion of the molded article to said surfaces by a physical process. The curing process of the molding composition is an aggressive chemical process which has the potential of involving the shape-determining surfaces of the mold to some extent, thereby further favoring adhesion of the molded article to said surfaces by a chemical process. For these reasons the molding of articles from curable liquid compositions presents release problems which are much more severe than those encountered during the molding of non-curing compositions, such as thermoplastic materials. Furthermore, in a closed-mold process, wherein the confined curing liquid composition may be subjected to pressure, such as in the preparation of high-resiliency polyurethane foam articles, such as automobile seat cushions, furniture upholstery and impact pads, said adhesion-favoring processes are accentuated.
For over twenty years silicone compositions have been used as mold release agents. However, some of the silicone compositions, such as polysiloxane waxes, greases and fluids, transfer to the molded article during demolding and need to be frequently reapplied to the mold. Other silicone compositions, while not being transferable, lack sufficient abrasion-resistance, either because they are not tough enough or because they do not adhere sufficiently to the mold, and frequently need to be repaired or reapplied in order to obtain molded articles having high fidelity.
Braley, U.S. Pat. No. 2,811,408, teaches that a copolymer of trimethylsiloxane units and SiO.sub.2 units, optionally containing an organopolysiloxane, provides excellent release of articles molded from organic and organosilicon plastics when applied to the surface of the mold. However, no provision is made for curing the optional organopolysiloxane, when used, and transfer thereof from the mold surface to the molded article will occur, thereby rendering the surface of the molded article unreceptive to further coating, such as painting. Uncured organopolysiloxane fluid, when used in a mold release composition, also has a tendency to defoam a polyurethane foam composition at the foam-fluid interface, resulting in an undesirable skin on the molded article.
Oppliger, Canadian Pat. No. 624,114, teaches that the use of a cured dimethylpolysiloxane film, adhered to the mold surface, represents an improved way for releasing thin-skinned polyurethane foam articles from metal or paper molds. While a cured dimethylpolysiloxane film is not likely to transfer to the molded article, such a film lacks sufficient abrasion resistance and does not provide improved release of several molded articles without frequent recoating of the mold.
Brooks, U.S. Pat. No. 3,684,756, discloses a resinous mold release composition which is resistant to abrasion and which comprises an alkyl-terminated organopolysiloxane, a partially alkoxylated silicone resin and a metal organic resin-curing catalyst. While this composition is said to give multiple releases of plastic articles from a mold treated therewith, the alkyl-terminated organopolysiloxane possesses no curing sites and is not cured in or co-cured with the resin. Transfer of the uncured alkyl-terminated organopolysiloxane from the mold surface to the surface of the molded article is likely, thus leading to the problems of transfer and defoaming discussed above.